The project is designed to determine the infective dose of Cryptosporidium parvum, a common waterborne pathogen, for antibody negative (previously unexposed) and antibody positive (previously exposed) persons. Strains of C. parvum from both human and animal origin will be evaluated in the model to determine interstrain variation in human pathogenicity. The feasibility of prevention of experimentally-induced infection will be examined using a bovine anti-cryptosporidium immunoglobulin preparation. Laboratory studies of the various Cryptosporidium strains will be undertaken in an attempt to define important human virulence characteristics. The volunteer data, used in computer models to look at risk assessment of the parasite for humans, will be considered in developing water quality standards for drinking water in the United States.